This invention relates to a hydraulically actuated preloaded connection between an annular body and a suspension member and a method for establishing the connection. More specifically, the invention relates to a preloaded connection between a wellhead body and a suspension joint which provides extended service life.
A non-preloaded connection between two members is one in which the members are structurally connected but not rigidly joined. Externally applied tensile forces are reacted by the direct axial attachment between the members but externally applied bending and shear loads are reacted by a socket action between the members. A force couple is created if one member rotates and contacts the other. An example of a non-preloaded structural connection used in industry is the connection made between a typical subsea wellhead landed in a suspension joint. The suspension joint on a cemented well casing forms the axial attachment to react production riser tension loads. The bending and shear loads applied to the wellhead by the riser are transferred to the structural casing by a force couple created if the wellhead lower body rotates within and contacts the suspension joint.
A preloaded connection between two members is one in which the members are rigidly joined by preloading the two members that typically comprise the connection. One member is preloaded in tension and the other member is preloaded in compression with a compressive preload force passing through the mating surfaces. Unlike a non-preloaded connection which relies on relative movement between the two members to create a force couple used in reacting to external bending and shear loads, the initial compressive force in a preloaded connection holds the members in contact when external loads are applied. Examples of typical preloaded connections used in industry are a bolted flange, a clamp/hub and a marine hydraulic connector. In these examples, the bolts of a bolted flange connection, the clamps of a clamp/hub connection and the body of a marine hydraulic connector are initially preloaded in tension. This results in the flanges, clamp hubs and the wellhead or similar hub, respectively, being placed in compression with an initial preload force passing through the mating surfaces.
As separating loads are applied to a preloaded connection, the majority of these separating loads go to relieve the initial compressive preload force at the mating surfaces. The remainder of the separating loads are reacted by the members of the connection originally preloaded in tension. If the separating loads are sufficient to totally relieve the initial compressive preload force, the mating surfaces break contact and all loads are reacted by the tensile members of the connection. When this occurs, the connection acts as a non-preloaded connection.
Because a preloaded connection serves to limit the amount of load actually reacted by the tensile members in the connection, due to compressive stress relief, the tensile stresses rise less rapidly than stresses in similar members of a non-preloaded connection. In addition, the alternating stresses produced from cyclic loads will typically be less for a preloaded connection than a non-preloaded connection of the same geometry. Therefore, the fatigue life of the preloaded connection will be greater than the non-preloaded connection.
A subsea wellhead installation includes a number of concentric tubular members. The outer tubular member is the structural casing whose upper portion includes a suspension joint for supporting the wellhead body. This casing is typically 30 inches (762 mm) in diameter. The wellhead body typically includes a 20 inch (508 mm) casing welded to its lower portion. A number of smaller diameter concentric casing members are suspended from the inner bore of the wellhead body. In the prior art, a wellhead body is connected to a suspension joint by a non-preloaded or "socket" type connection. A wellhead for tension leg platforms may be required to withstand large external loads from drilling operations, ocean currents and storm conditions. If the wellhead is tied back to a vessel with a production riser, a large number of smaller amplitude cyclical fatigue loads must be withstood. Because the prior art wellhead body and suspension joint are not rigidly joined (preloaded), much of these loads are not directly transferred to the larger and stronger 30 inch (762 mm) casing. Instead, they are transferred into the 20 inch (508 mm) casing as the wellhead body rotates in its "socket". Accordingly, the stress from the external loads, particularly the large number of cyclical loads, can cause fatigue failure to the 20 inch (508 mm) casing suspended from the wellhead body.
It is known to use preloaded wellhead connectors utilizing split locking rings on subsea wellheads. However, these connectors require integral hydraulics to actuate the locking rings. These connectors must be bored for the hydraulic fluid passages, machined and include many components associated with the actuating mechanism.
I have determined that a preloaded connection can be used to connect an annular body to a suspension member by expanding a split ring between the annular body and the suspension member using an actuating ring. Integral hydraulics are not required in the connection to operate the actuating mechanism. When expanded by the actuating ring, the locking ring develops sufficient compressive preload force on the annular body and tensile preload force on the suspension member to maintain the mating surfaces together throughout the expected life of the annular body. Since the externally applied loads do not exceed the compressive preload, the external loads are transferred into the suspension members thereby reducing or eliminating chances for fatigue failure in the annular body or the casing there below. For example, a preloaded wellhead can now be expected to remain in service for its 30-year design life.